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Old 12-08-2021, 11:01 AM   #20
Dannyx
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Join Date: Aug 2016
City & State: Constanta
My Country: Romania
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Posts: 3,248
Red face Re: Master power switch for E-bike ideas

Ok, I'm currently looking to grab one of those big FETs for this project along with some other stuff from a supplier I often order electronic stuffz from. I mean it's not like there's any urgency to it since it's friggin' winter and the bike is out of service for half a year...

I WAS gonna do that, but then I thought I should take a step back and analyze what it is that I really need....and it's also here I discovered I don't know a lot of things about FETs, or at least some things I THOUGHT were clear ended up getting confusing.

For instance this: I'm not sure what Vds means for my simple switch application. For the same price, I have a choice of two of those beefy boyz: THIS one which does 60v Vds and 400A or THIS one which does 100v Vds, but only 320A AND has a higher RdsOn. Realistically, I don't think I'll EVER draw that much current, unless maybe in very sharp pulses, so I aimed for the one with the lowest RdsOn. What does Vds mean in practical terms ? I tried reading about it as much as possible and I'm pretty sure it refers to the drop across D-S of the FET, regardless of whether it's on or off. In practice, which is what I care most about, I put my meter probes across D-S: if the FET is off (Vgs is 0), then Vds=VBat (?) because it's like an open switch ? If the FET is "on", Vds drops to nearly 0 because the "switch" is closed and conducting, having very little resistance of its own (RdsOn)?

This makes sense and I understand it, but I'm struggling to interpret the SOA graphs. Take my soft-start circuit I'm trying to create, does this mean that Vds starts off high at Vbat when the FET is off, no matter what, and slowly goes down as the transistor opens up more and the caps charge ? Based on the SOA graph, what happens AFTER the transistor is on ? Since Vds is now low, does this mean I can safely draw up to 100A through it ?

Then there's that high gate capacitance I need to address, but let's start with the basics. Feel free to enlighten me, as I feel ashamed for not being able to figure this out
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